Process for X-ray mask warpage reduction

Abstract

An X-ray mask substrate includes a silicon wafer having a square, central region etched to a thin, tensile membrane and a highly tensile film deposited on the bottom surface of the substrate to reduce substrate warpage. The square, central region of the substrate is adapted to support X-ray absorbing material during the lithography process. A layer of highly tensile film such as tungsten is deposited on the lower side of the substrate to induce a bending moment on the substrate opposite that induced during the substrate fabrication process. The thickness of the film layer is directly proportional to the amount of warpage induced in the substrate during the fabrication process. A support ring is bonded to the peripheral region of the substrate to provide integrity and support.

What is claimed is: 
     
       1. An exposure mask for use in a lithography process, comprising: a substantially X-ray opaque body having a central area and substantially parallel top and bottom surfaces,   said central area on said bottom surface having an X-ray translucent mask region substantially parallel with said top and bottom surfaces,   said body being normally warped with respect to the top surface by internal forces present during formation of the X-ray translucent mask region,   a layer of film deposited on said bottom surface in the area surrounding said central area, said film selected to apply compensating tensile forces when deposited on said body which reduce the warpage caused by the internal forces present during formation of the mask region.   
     
     
       2. An exposure mask as in claim 1, wherein said X-ray opaque body comprises a monocrystalline silicon wafer. 
     
     
       3. An exposure mask as in claim 1, wherein said X-ray translucent mask region in said central area on said bottom surface includes a recess. 
     
     
       4. An exposure mask as in claim 3, wherein said recess forms a thin tensile membrane adapted to provide increased X-ray transmission. 
     
     
       5. An exposure mask as in claim 4, wherein said body in the central area is about 0.0025 mm thick, and said body surrounding the central area is about 0.625 mm thick. 
     
     
       6. An exposure mask as in claim 1, wherein said body is normally warped concave with respect to the top surface by internal forces present during formation of the X-ray translucent mask region. 
     
     
       7. An exposure mask as in claim 6, wherein said film induces a bending moment opposite that induced during the formation of said X-ray translucent mask region. 
     
     
       8. An exposure mask as in claim 7, wherein the film is a highly tensile tungsten film. 
     
     
       9. An exposure mask as in claim 8, wherein the thickness of said film is between about 100 Å and about 10,000 Å. 
     
     
       10. An exposure mask as in claim 9, wherein the film is deposited in a uniform layer across the bottom surface. 
     
     
       11. An exposure mask as in claim 9, wherein the film is deposited in a gradient across said bottom surface. 
     
     
       12. An exposure mask as in claim 1, wherein said bottom surface of said body has a portion which is bonded to a support ring. 
     
     
       13. An exposure mask as in claim 12, wherein said support ring is bonded directly to the bottom surface of said body. 
     
     
       14. An exposure mask as in claim further comprising a layer of X-ray absorbing material selectively deposited in a predetermined pattern on said top surface of said body. 
     
     
       15. An exposure mask as in claim 14, wherein said X-ray absorber material is selectively deposited to provide equalization of the pattern densities across the body. 
     
     
       16. An exposure mask as in claim 15, wherein said X-ray absorbing material is gold, tungsten or tantalum 
     
     
       17. A method for fabricating an X-ray mask substrate, comprising: forming an X-ray translucent mask region in a central area of a substantially planar substrate having a top and bottom surface, the formation of said X-ray translucent mask region inducing a bending moment which causes a warped configuration of said substrate with respect to the top surface,   selectively applying a layer of highly tensile film to the bottom surface of the substrate in the area surrounding the central region,   selecting the thickness of the film layer to induce a bending moment in the substrate opposite that induced during the formation of the X-ray translucent mask region to provide a more planar and less-warped substrate having an X-ray translucent mask region.   
     
     
       18. A method as in claim 17 wherein the substrate is monocrystalline silicon. 
     
     
       19. A method as in claim 17, wherein a recess is formed by etching the central region of the substrate from the bottom surface, said recess forming a thin tensile membrane adapted to provide increased X-ray transmission. 
     
     
       20. A method as in claim 19, wherein a dopant is selectively diffused into at least one surface of the substrate, said dopant providing an etch-stop during the etching process. 
     
     
       21. A method as in claim 20, wherein the dopant is boron. 
     
     
       22. A method as in claim 17, wherein the layer of film is tungsten. 
     
     
       23. A method as in claim 22, wherein the tungsten film is applied in a uniform layer. 
     
     
       24. A method as in claim 22, wherein the tungsten film is selectively applied in a gradient across the bottom surface of the substrate in the area surrounding the central region. 
     
     
       25. A method as in claim 17 wherein the layer is applied in multiple steps, and wherein the amount of warpage is measured after at least one step. 
     
     
       26. A method as in claim 25, wherein the thickness of the film initially applied in the first step to the bottom surface of the substrate is determined by the equation: ##EQU2## where: W=desired warpage correction (measured as the absolute value of the maximum difference in heights of the measured surface); ν s  =substrate material Poisson's ratio;   E s  =substrate material modulus of elasticity;   t f  =film thickness;   t s  =substrate thickness;   d=substrate diameter;   σ=film stress as measured with respect to particular deposition parameters.   
     
     
       27. A method as in claim 17, wherein the formation of said X-ray translucent mask region induces a bending moment which causes a warped configuration of said substrate concave with respect to the top surface. 
     
     
       28. A method as in claim 17, wherein a support ring is bonded directly to the peripheral region of the substrate, and X-ray absorbing material is applied to the top surface of the substrate in the central region after the film is applied. 
     
     
       29. A method as in claim 28, wherein said X-ray absorbing material is selectively deposited in a predetermined pattern on the top surface of said substrate. 
     
     
       30. A method as in claim 29, wherein said X-ray absorbing material is selectively deposited on the top surface of said substrate to provide an equalization of the pattern densities across the substrate. 
     
     
       31. A method for exposing selected patterns on a photoresist-covered semiconductor wafer with X-rays during a lithographic process, comprising the steps of: forming an X-ray translucent window in a substantially X-ray opaque substrate having a top and bottom surface, the formation of said X-ray translucent window inducing a bending moment which causes a warped configuration of said substrate with respect to the top surface,   selectively applying a layer of highly tensile film to the bottom surface of the substrate in the area surrounding the central region,   selecting the thickness of the layer to induce a bending moment in the substrate opposite that induced during the formation of the X-ray translucent window to provide a more planar and less-warped substrate having an X-ray window,   bonding a support ring to the periphery of the bottom surface of the substrate,   selectively depositing an X-ray absorbing material in the central region on the top surface of the substrate,   locating the substrate proximate the photoresist-covered semiconductor wafer,   directing the X-rays through apertures formed by the X-ray absorbing material on the substrate to expose selected patterns on the photoresist-covered semiconductor wafer.   
     
     
       32. In an exposure mask for use in a lithography process having a substantially X-ray opaque body with a central area and substantially parallel top and bottom surfaces, a recess formed in the central area on the bottom surface forming a thin X-ray transparent mask region and normally warping the body concave with respect to the top surface by internal forces present during formation of the recess, the improvement comprising a layer of film deposited on the bottom surface of the body in the area surrounding the central area, said film selected to apply compensating tensile forces when deposited on the body which reduce the warpage caused by the internal forces present during formation of the recess.